Myostatin (MSTN) is a secreted protein that plays an important role in regulating muscle mass. We originally identified myostatin in a screen for new members of the transforming growth factor-ss (TGF-ss) super family in mammals. We showed that Mstn is expressed specifically in the skeletal muscle lineage both during embryonic development and in adult mice and that targeted deletion of the Mstn gene in mice leads to a dramatic and widespread increase in skeletal muscle mass. Subsequent genetic studies in cattle, sheep, dogs, and humans have all shown that the function of myostatin as a negative regulator of muscle mass has been highly conserved across species. The demonstration that myostatin normally acts to limit muscle mass has suggested the possibility that targeting the myostatin pathway may have utility for enhancing muscle growth and regeneration in disease states characterized by debilitating muscle loss, including muscle degenerative diseases, neuromuscular diseases, cachexia, and age-related sarcopenia. Indeed, a number of studies have demonstrated beneficial effects of targeting the myostatin pathway in many of these disease settings. In order to develop strategies and methods for exploiting this signaling pathway for human therapeutic applications, we have focused much of our work on understanding the mechanisms by which myostatin signals to target cells and by which myostatin activity is regulated. The overall goal of this project is to continue our efforts to elucidate the molecular and cellular mechanisms underlying myostatin action. A major goal of this project will be to identify the cell types in muscle that are the direct targets for myostatin signaling in vivo. There is considerable debate as to whether myostatin normally exerts its effect by signaling to satellite cells, which are the stem cells resident in muscle, or directly to myofibers. In the first part of this project, we will attempt to determine the role of satellite cells in mediating myostatin signaling and the effects of myostatin inhibition. These studies will be important not only for understanding the basic biology of skeletal muscle growth but also for pursuing clinical applications based on targeting this pathway, as a critical question has been whether therapies based on myostatin inhibition will have beneficial effects in disease settings where the satellite cell population has already been depleted. In the second part of this project, we will continue our efforts to understand the roles of key components of this regulatory system. In particular, we will use genetic approaches in mice to characterize further the role of activin type II receptors in mediating myostatin signaling and the role of follistatin in regulating myostatin activity. Taken together, we believe that the results of these studies will provide important insights into the mechanism of action of myostatin and its regulatory components and that these findings could have important implications both for assessing which disease states might be most responsive to therapeutic agents targeting this pathway and for identifying the most effective strategies for therapeutic intervention.